Development of communication technology, etc. in recent years makes it possible to provide information display devices for use in personal computers, portable information terminals and the like which can switch between the display of dynamic images and static images in accordance with the signal received This makes it possible to provide various kinds of information depending upon the user's needs and preference. However, in such an image display device, electricity consumption is increased because an on-off action is repeated whenever a dynamic image is displayed. A portable information terminal uses a battery as a driving power source thereof, and therefore reducing the amount of energy used is a particularly important object because an increase in energy consumption results in a shortened usage duration per charge.
A method for reducing the number of bits of image data has been proposed for reducing energy consumption in image display devices. As an example of an image display device provided with an image processing device which performs such processing, FIG. 4 shows a structure disclosed in Japanese Unexamined Patent Publication No. 1997-101771.
As shown in the figure, an image display device 50 comprises a video controller 51 for controlling the image display device 50, a VRAM 52 for storing RGB image data, a liquid crystal display 53 for displaying the RGB image data and a liquid crystal controller 54 for controlling the liquid crystal display 53. The image display device 50 is connected to a personal computer 56 and an image processing device 58. When RGB image data is input from a hard disk drive (not shown) of the personal computer 56. the image processing device 58 reduces the size of the image data by masking the lowest three bits thereof which are nearly uninfluential on the quality of the display image, conducts scaling and subtractive color processing, and then outputs the image data to the image display device 50. As the subtractive color process, as disclosed in the above publication, a dither method, an error diffusion method and the like are known. The RGB image data input into the image display device 50 is transferred to the VRAM 52 and displayed on the liquid crystal display 53 controlled by the liquid crystal controller 54.
The above-described image display device saves energy by reducing the number of bits of the RGB image data in the image processing device 58; however, the image processing device 58 continues to operate while an image is displayed on the liquid crystal display 53 regardless of whether the image is dynamic or static. Therefore. there is room for further improvement in energy reduction.